Get the total time spent working on a file
There's an addon that keeps track of the actual time you spent working on the project.



http://wiki.blender.org/index.php/Extensions:2.6/Py/Scripts/System/Project_Timer


  The time spent working in the blend file is recorded, and stored in
  the blend to continue from the next time it is opened.
  
  If you stop work for longer than 30 seconds, the timer stops counting
  and reverts back to the time of your last action, minimizing idle
  time.


Edit: Download link seems to no longer work, here's the last version of the add-on I could find: Download (unzip, install py file).